The present invention relates to an information recording medium which is reproducible by utilizing near-field light and an information reproducing apparatus for reproducing information recorded on the information recording medium at high density, particularly to an information recording medium and an information reproducing apparatus enabling tracking control.
Currently, many of information reproducing apparatus carry out reproduction of information with an information medium of a magnetic disk or an optical disk as an object, particularly, CD (Compact Disk) which is one of optical disks is widely used as a medium enabling information recording at high density and mass production at low cost and recording a large capacity of information. A surface of CD is formed with pits each having a size of about a wavelength of laser beam used in reproduction and a depth of about a quarter of the wavelength and reading operation utilizing interference phenomenon of light is carried out.
In reading recorded information from an optical disk represented by CD, generally, there is utilized a lens optical system used in an optical microscope. In this case, when an information recording density is increased by reducing a size of the pit or a track pitch thereof, a deadlock is reached by the problem of diffraction limit of light in which a spot size of laser beam cannot be made equal to or smaller than a half wavelength and an information recording unit cannot be reduced to a size smaller than the wavelength of laser beam.
Further, not only in CD but also in optical recording disks recorded with information by a magneto-optical recording system and a phase change recording system, record and reproduction of information at high density is realized by a very small spot of laser beam and accordingly, the information recording density is restricted by a diameter of the spot provided by converging laser beam.
Hence, in order to break the restriction imposed by the diffraction limit, there is proposed an information reproducing apparatus using an optical head provided with a very small aperture having a diameter equal to or smaller than a wavelength of laser beam utilized for reproduction, for example, a diameter of about {fraction (1/10)} of the wavelength and utilizing near-field light (including both of near-field light and far-field light) formed at the very small aperture portion.
Originally, as an apparatus utilizing near-field light, there is provided a near-field microscope using a probe having the above-described very small aperture and the near-field microscope is utilized in observing a very small surface structure of a sample. As one of systems of utilizing near-field light in a near-field microscope, there is provided a system in which a very small aperture of a probe and a surface of a sample are made proximate to each other such that a distance therebetween is equal to about a diameter of the very small aperture of the probe and by introducing propagating light via the probe toward the very small aperture of the probe, near-field light is formed at the very small aperture (illumination mode). In this case, scattered light produced by interaction of the formed near-field light and the surface of the sample, is detected by a scattered light detecting system with an intensity and a phase reflecting a fine structure of the surface of the sample and there is enabled observation having high resolution which cannot be realized by a conventional optical microscope.
Further, as other system of a near-field microscope utilizing near-field light, there is provided a system in which a sample is irradiated with propagating light to thereby localize near-field light on the surface of the sample and a very small aperture of a probe is made proximate to the surface of the sample to a degree of a diameter of the very small aperture of the probe (collection mode). In this case, scattered light produced by interaction of the localized near-field light and the very small aperture of the probe, is guided to a scattered light detecting system via the very small aperture of the probe with an intensity and a phase reflecting a fine structure of the surface of the sample to thereby achieve observation with high resolution.
According to the above-described information reproducing apparatus utilizing near-field light, there are utilized these observation systems in the near-field microscope and by unitizing the near-field light, there is enabled reproduction of information of an information recording medium recorded with information with higher density.
In order to realize reproduction of information recorded on the information recording medium with high density by utilizing the above-described near-field light, there is needed a positioning control technology for moving the very small aperture portion of the probe constituting an optical head to an arbitrary position on the information recording medium with high precision.
In the case of a magnetic disk apparatus, generally, as positioning control, there are adopted a servo face servo system and a sector servo system. The servo face servo system is a method in which one face in a plurality of disk faces is used exclusively for servo, a magnetic head for servo is positioned relative to the servo face and a remaining disk face and a magnetic head are used for data. Further, the sector servo system is a method in which servo information is embedded sporadically on data face and by using the servo information detected discretely, a magnetic head is positioned on data tracks.
However, it is difficult to apply positioning control adopted in these magnetic disk apparatus to positioning control in respect of reproduction of a high density information recording medium by near-field light. For example, according to the above-described servo face servo system, positional accuracies of a head for servo and a head for data are determined by mechanical accuracies and accordingly, there is a case in which a positional shift is produced between the both heads by a difference in temperature distribution and the system is not suitable for being adopted as positioning control in respect of an information recording medium with high density formation.
Further, according to the above-described sector servo system, although there causes no positional shift of heads caused by the difference in the temperature distribution which is problematic in the servo face servo system, at a design stage of a control system, the control system needs to deal with as a discrete value system different from a conventional continuous system and in the case of an information reproducing apparatus utilizing near-field light, there is needed high accuracy positioning with regard to an information recording medium with particularly high density formation and accordingly, it is not preferable to use such a complicated control system.
Meanwhile, according to an optical disk apparatus, as positioning,control methods, particularly as tracking error detecting methods, there are adopted a three beam method, a push-pull method and a prewobbling tracking error detecting method. The three beam method is a method in which beam from a laser diode is divided into a total of three beams of a 0-th order beam (main beam) for record and reproduction and two 1-th order beam (sub beam) for tracking by a diffracting grating, two of the sub beams are slightly shifted from a center of a guide groove provided on an optical disk, fluxes of reflected beam from both of them are received by two light receiving faces of an optical detector and an objective lens is controlled by a differential signal thereof.
Further, the push-pull method is a method in which fluxes of reflected beam of beam irradiated to guide grooves provided on an optical disk are detected by a two split detector and a differential signal provided thereby constitutes a track error signal to thereby control an objective lens. The prewobbling tracking error detecting method is a method in which a set of two long pits (prewobbling marks) A and B are previously arranged on an optical disk to slightly shift from each other in the disk radius direction relative to a center of a track and a change in an amount of fluxes of reflected beam from the pits A and B produced in tracing the center of the track by a light spot, constitutes a track error signal to thereby control an objective lens.
The above-described tracking error detecting methods of the optical disk apparatus are methods in the case in which both of irradiated light to the pit formed on the optical disk and reflected light reflected thereby are dealt with as propagating light (far-field) and a devise is needed when the methods are applied to detection of nonpropagated beam (near-field) such as near-field light and reflected and scattered light thereof. Further, particularly, in a case of an information recording medium enabling reproduction by utilizing near-field light, an information recording unit can be determined not only by recess and projection information as in a pit formed on a conventional optical disk but also a difference in an optical property and accordingly, there is requested an information reproducing apparatus for carrying out optical head positioning control, particularly, tracking in order to reproduce such an information recording medium.
It is an object of the invention to provide an information recording medium and an information reproducing apparatus for realizing reproduction of information having high reliability in respect of an information recording medium recorded with information at high density, particularly, tracking by a simple constitution.
In order to achieve the above-described object, according to a first aspect of the invention, there is provided an information recording medium characterized in that in an information recording medium in which information to be reproduced by a reproducing probe provided with a very small aperture for forming near-field light is formed on a surface of the medium, wherein a unit of the information is constructed by a structure in which inclined faces or curved faces are provided symmetrically in a left and right direction relative to a reading direction and the left and right inclined faces intersect with each other or the left and right curved faces coincide with each other.
According to the first aspect of the invention, the unit of information is constructed by the structure in which the inclined faces or the curved faces are provided symmetrically in the left and right direction with the reading direction as the central axis and accordingly, directionalities can be given to fluxes of reflected light which are reflected when the unit of information is irradiated with fluxes of light and a difference between intensities of the fluxes of reflected light can be unitized as a signal for tracking.
Further, according to a second aspect of the invention, there is provided the information recording medium according to the first aspect, characterized in that the unit of the information is constituted by a section orthogonal to the reading direction in a triangular shape.
According to the second aspect of the invention, the section of the unit of information orthogonal to the reading direction is formed in the triangular shape and accordingly, when the unit of information is irradiated with fluxes of light, the fluxes of light can efficiently be reflected at the inclined faces of the unit of information, further, the directionalities can be given to fluxes of the reflected light and accordingly, the difference between the intensities of the fluxes of reflected light can be utilized as a signal for tracking.
Further, according to a third aspect of the invention, there is provided the information recording medium according to the first aspect, characterized in that the unit of the information is constituted by a section orthogonal to the reading direction in a semicircular shape.
According to the third aspect of the invention, the section of the unit of the information orthogonal to the reading direction is formed in the semicircular shape and accordingly, when the unit of information is irradiated with fluxes of light, the fluxes of light can efficiently be reflected at the curved faces of the unit of the information, further, the directionalities can be given to fluxes of the reflected light and accordingly, the difference between the intensities of the fluxes of reflected light can be utilized as a signal for tracking.
Further, according to a fourth aspect of the invention, there is provided the information recording medium according to the second or the third aspect, characterized in that the unit of the information is formed in a convex relative to a surface of the medium.
According to the fourth aspect of the invention, the unit of information is constructed by a structure in which the unit of the information is provided with the inclined faces or the curved faces symmetrically in the left and right direction with the reading direction as the central axis and is formed in the convex relative to the surface of the information recording medium and accordingly, when the unit of the information is irradiated with fluxes of light which are shifted on the left side, the fluxes of light can be reflected strongly in the left direction and when the unit of the information are irradiated with fluxes of light which are shifted on the right side, the fluxes of light can be reflected strongly in the right direction and accordingly, the difference between the intensities of, fluxes of reflected light can be utilized as a signal for tracking.
Further, according to a fifth aspect of the invention, there is provided the information recording medium according to the second or the third aspect, characterized in that the unit of the information is formed in a concave relative to a surface of the medium.
According to the fifth aspect of the invention, the unit of information is constructed by a structure in which the unit of the information is provided with the inclined faces or the curved faces symmetrically in the left and right direction with the reading direction as the central axis and is formed in the concave relative to the surface of the information recording medium and accordingly, when the unit of information is irradiated with fluxes of light which are shifted to the left, the fluxes of light can be reflected strongly in the right direction and when the unit of information is irradiated with fluxes of light which are shifted to the right, the fluxes of light can be reflected strongly in the left direction and accordingly, the difference between the intensities of fluxes of the reflected light can be utilized as a signal for tracking.
Further, according to a sixth aspect of the invention, there is provided the information recording medium according to any one of the first through the fifth aspects, characterized in that a metal reflecting film is formed on the surface of the medium.
According to the sixth aspect of the invention, the metal reflecting film is formed on the surface of the information recording medium and accordingly, fluxes of light irradiated to the unit of information can efficiently be reflected.
Further, according to a seventh aspect of the invention, there is provided an information reproducing apparatus for reproducing information by a reproducing probe provided with a very small aperture for forming near-field light, characterized in comprising an information recording medium in which a unit of the information is constructed by a structure in which inclined faces or curved faces are provided symmetrically in a left and right direction relative to a reading direction and the left and the right inclined faces intersect with each other or the left and the right curved faces coincide with each other, and a control apparatus for detecting fluxes of reflected and scattered light produced by scattering the near-field light by the unit of the information at at least two positions symmetrical with each other relative to a central axis of the very small aperture along the reading direction and controlling a position of the reproducing probe in accordance with a detected signal.
According to the seventh aspect of the invention, by forming the unit of the information constructed by the structure in which the inclined faces or the curved faces are provided on the left and on the right with the reading direction as the central axis and with the central axis as a top portion, to the information recording medium and making the near-field light formed at the very small aperture of the reproducing probe incident on the information unit, there can be formed fluxes of the reflected and, scattered light provided with the directionalities to the left and to the right, the fluxes of reflected and scattered light are detected at at least two-positions symmetrical with each other relative to the central axis of the very small aperture and the position of the reproducing probe is controlled in accordance with the detected signal and accordingly, tracking control with high accuracy utilizing the near-field light can be carried out.
Further, according to an eighth aspect of the invention, there is provided an information reproducing apparatus for reproducing information by a reproducing probe provided with a very small aperture for forming near-field light, characterized in comprising an information recording medium in which a unit of the information is constructed by a structure in which inclined faces or curved faces are provided symmetrically in a left and right direction relative to a reading direction and the left and the right inclined faces intersect with each other or the left and the right curved faces coincide with each other and a first and a second light detecting means arranged symmetrically with each other in the left and right direction relative to a central axis of the very small aperture along the reading direction for detecting fluxes of reflected and scattered light produced by scattering the near-field light by the unit of the information and outputting detected signals, difference calculating means for calculating a difference between a first detected signal outputted from the first light detecting means and a second detected signal outputted from the second light detecting means and outputting a differential signal, reproducing probe position controlling means for controlling a position of the reproducing probe in accordance with the differential signal, and reproduced signal forming means for calculating to add the first detected signal and the second detected signal and forming a reproduced signal.
According to the eighth aspect of the invention, by forming the unit of the information constructed by the structure in which the inclined faces or the curved faces are provided on the left and on the right with the reading direction as the central axis and with the central axis as a top portion, in the information recording medium and making the near-field light formed at the very small aperture of the reproducing probe incident on the unit of the information, there can be formed fluxes of the reflected and scattered light provided with the directionalities to the left and to the right, the fluxes of reflected and scattered light are detected by the first and the second light detecting means arranged at at least two positions symmetrical with each other relative to the central axis of the very small aperture, the differential signal indicating the difference between the first detected signal detected and outputted by the first light detecting means and the second detected signal detected and outputted by the second light detecting means, is formed by the difference calculating means, positioning control of the reproducing probe in accordance with the differential signal can be carried out by the reproducing probe position controlling means and accordingly, tracking control with high accuracy utilizing the near-field light can be carried out. Further, the reproduced signal can be formed by calculating to add the first detected signal and the second detected signal and accordingly, reproduction of the information can simultaneously be carried out.
Further, according to a ninth aspect of the invention, there is provided the information recording apparatus according to the seventh or the eighth aspect, characterized in that the reproducing probe comprises an optical fiber provided with the very small aperture at a front end thereof.
According to the ninth aspect of the invention, as the reproducing probe, there can be utilized a probe of an optical fiber type used in a conventional near-field microscope and accordingly, accumulated technology of the near-field microscope can effectively be applied to the information reproducing apparatus.
Further, according to a tenth aspect of the invention, there is provided the information reproducing apparatus according to the seventh or the eighth aspect, characterized in that the reproducing probe is a probe of a cantilever type provided with the very small aperture at a projected portion thereof.
According to the tenth aspect of the invention, as the reproducing probe, there can be utilized the probe of the cantilever-type used in the conventional near-field microscope and accordingly, the accumulated technology of the near-field microscope can effectively be applied to the information reproducing apparatus.
According to an eleventh aspect of the invention, there is provided the information recording apparatus according to the seventh or the eighth aspect, characterized in that the reproducing probe is a plane probe comprising a plane substrate formed by being penetrated with a hole in a shape of an inverse cone to constitute the very small aperture by a top portion thereof and the first and the second light detecting means are arranged at the plane substrate.
According to the eleventh aspect of the invention, as the reproducing probe, there can be utilized the plane probe arranged with the first and the second light detecting means and accordingly, a simple apparatus constitution can be realized with no need of adjusting positions of the first and the second light detecting means.
Further, according to a twelfth aspect of the invention, there is provided the information reproducing apparatus according to the eleventh aspect, characterized in that the reproducing probe is the plane probe arranged with a third and a fourth light detecting means for detecting the fluxes of the reflected and scattered light at vicinities of the very small aperture.
According to the twelfth aspect of the invention, as the reproducing probe, there can be utilized the plane probe arranged with the first and the second light detecting means and the third and the fourth light detecting means for detecting fluxes of the reflected and scattered light at vicinities of the very small apertures and accordingly, the reproduced signal having sufficient intensity can be detected by the first and the second light detecting means and the third and the fourth light detecting means.
Further, according to a thirteenth aspect of the invention, there is provided an information reproducing apparatus for forming near-field light at an information recording medium and reproducing information by a reproducing probe for scattering the near-field light, characterized in that fluxes of reflected and scattered light produced by scattering the near-field light produced at a unit of the information formed on the information recording medium by a front end portion of the reproducing probe, are detected at at least two positions symmetrical with each other relative to the front end portion of the reproducing probe and a position of the reproducing probe is controlled in accordance with a detected signal.
According to the thirteenth aspect of the invention, the near-field light produced at the unit of information of the information recording medium is scattered by the reproducing probe, fluxes of the reflected and scattered light are detected at at least two positions symmetrical with each other relative to the front end portion of the reproducing probe, the position of the reproducing probe is controlled in accordance with the detected signal and accordingly, tracking control with high accuracy utilizing the near-field light can be carried out.
Further, according to a fourteenth aspect of the invention, there is provided an information reproducing apparatus for forming near-field light at an information recording medium and reproducing information by a reproducing probe for scattering the near-field light, characterized in comprising a first and a second light detecting means arranged symmetrically with each other relative to a front end portion of the reproducing probe along a reading direction for detecting fluxes of reflected and scattered light produced by scattering the near-field light by a front end portion of the reproducing probe and outputting detected signals, difference calculating means for calculating a difference between a first detected signal outputted from the first light detecting means and a second detected signal outputted from the second light detecting means and outputting a differential signal, reproducing probe position controlling means for controlling a position of the reproducing probe in accordance with the differential signal and reproduced signal forming means for forming a reproduced signal by calculating to add the first detected signal and the second detected signal.
According to the fourteenth aspect of the invention, the near-field light produced at the unit of the information of the information recording medium is scattered by the reproducing probe, fluxes of the reflected and scattered light are detected by the first and the second light detecting means arranged at at least two positions symmetrical with each other relative to the central axis of the very small aperture, the differential signal indicating the difference between the first detected signal detected and outputted by the first light detecting means and the second detected signal detected and outputted by the second light detecting means, is formed by the difference calculating means, position control of the reproducing probe in accordance with the differential signal can be carried out by the reproducing probe position controlling means and accordingly, tracking control with high accuracy utilizing the near-field light can be carried out. Further, the reproduced signal can be formed by carrying out operation of adding the first detected signal and the second detected signal by the reproduced signal forming means and accordingly, the information can simultaneously be reproduced.
Further, according to a fifteenth aspect of the invention, there is provided the information reproducing apparatus according to the thirteenth or the fourteenth aspect, characterized in that the reproducing probe is formed in a wedge-like shape.
According to the fifteenth aspect of the invention, the reproducing probe is formed in the wedge-like shape and accordingly, by reflecting the near-field light produced at the unit of the information of the information recording medium by the two inclined faces constituting the wedge-like shape, the directionalities can be given to fluxes of the reflected light and accordingly, the difference between the intensities of the fluxes of reflected light can be provided further significantly and tracking control with high accuracy utilizing the near-field light and having high reliability can be carried out. Further, sufficiently large intensity can be provided to the reproduced signal by the fluxes of reflected light.